Many Guardsmen and Reservists served as individuals or as aircrews but only one Guard unit that I know of (a Long Range Recon Patrol Company from the Indiana Army Nat'l Guard) was called up as a political message following Tet 68. You must remember that our national position was that we had to maintain a "credible" force as a hedge against Soviet/surrogate action elsewhere, that many state governors were reluctant to have their Guard troops called away in the face of actual and potential civil unrest, and that President Johnson was reluctant to call up the reserve components (Guard & Reserve) because of the political message it would send.
This latter reason was the impetus behind the "Total Force" policy that guided our national military planning and force structure decisions following the Vietnam War. Many who became senior officers consciously organized the military so that it would be impossible for another president to commit the nation to another Vietnam-like conflict WITHOUT A SUBSTANTIAL CALL-UP OF THE RESERVE COMPONENTS -- because that is how the president asks the American people if they want to go to war.
This policy worked extremely well during the Gulf War.
The whole situation is best illustrated by the truism that "when the reserve components go to war they take with them in their rucksacks something that the active components don't really have in theirs -- the committment of the Americian public!"
And for Just A Joke and his/her insulting remark about the reserve components: While there are some who take reserve component service lightly, this is by no means the case for all. In my 30 years as a Reservist I was trained not only as an infantryman but also as a paratrooper, ranger and "Green Beret." I completed the advanced officer courses of both the Infantry and Armor Schools as well as the Command and General Staff College and Army War College courses. I served over half of my 30 years on active duty, including over 27 months in combat where I led/commanded infantry platoons, companies and battalions. I have been awarded the Silver Star, two Bronze Stars, and a footlocker full of Meritorious Service and Commendation Medals. When not actually on active duty, I devoted (at least) one night per week, two weekends per month, and four weeks per year to my duties -- all while holding down a good full-time civilian job and having a family.
As in any pursuit, some give only the minimum and let others pull their weight for them while they stand on the sidelines and complain/make jokes. If you truly think the reserve components are a joke, then I suggest you join them and work to make them worthy ("come with me to Macedonia").

Somewhere lost in all the namecalling was an answer to the original question. I served on active duty in Vietnam from 1969-70. There was a unit of the Kentucky National Guard in Phu Bai, outside of Hue, where I was stationed. It was the 2/138th Artillery. As I recall, they took a lot of casualties at Hai Van pass. I was told that they were the only combat unit activated for the war, but that was just hearsay. They went home in 1969 and were replaced by an active duty Artillery battalion. They used self propelled 155 howitzers. Hope this helps.

On May 13, 1968, 12,234 Army National Guardsmen in 20 units from 17 states were mobilized for service during the Vietnam War. Eight units deployed to Vietnam and over 7,000 Army Guardsmen served in the war zone. Company D (Ranger), 151st Infantry, Indiana Army National Guard arrived in Vietnam in December 1968. As part of the II Field Force, the Indiana Rangers were assigned reconnaissance and intelligence-gathering missions. Operating deep in enemy territory, Ranger patrols engaged enemy units while conducting raids, ambushes and surveillance missions. "Delta Company" achieved an impressive combat record during its tour in Vietnam; unit members were awarded 510 medals for valor and service. The gallant record of Company D, 151st Infantry symbolizes the Army National Guard's performance in Vietnam.

Someone needs to better document this. A few years ago I came across a magazine artical claiming that only some support units were activated (in Hawaii) and none were sent ot Viet Nam. The dates given for those units activation were in 1967.
That small 1967 group and the 12,000 guards described as activated in 1968 'to send a message' sounds typical of the incremental "escalation" strategy of MacNamarra.

Ben, I can still clearly hear my Drill Sgt. reminding us reservists that we would not be overlooked in the event of a war. He also said that one of the first US units in Viet nam was from the Oklahoma National guard. If you GOOGLE "national Guard Units in Viet Nam" you will get a list of 17 guard units that were activated. I was often referred to as a weekend warrior by active duty military that I met while at annual training or MUTA. I had to chuckle because as a Deputy Sheriff I carried loaded weapons all month and unloaded weapons while on miltary duty. VO 91B30,91D30,38A SSG AUS ret.